Such a process is known from WO 01/73173 A1. A polyethylene multifilament yarn with a tensile strength of 4.0 GPa for a yarn containing 60 filaments is decribed in this patent publication, which was made by a continuous process comprising the steps of    a) making a solution of 8 mass % of ultra-high molar mass polyethylene homopolymer having an intrinsic viscosity of 27 dl/g in mineral oil;    b) spinning of the solution through a spinplate containing 60 spinholes, each having a tapered inflow zone of unspecified dimension and a downstream zone of about 1 mm diameter and length/diameter ratio (L/D) of 40, into an air-gap of about 3.2 mm to form fluid filaments, while applying a draw ratio DRfluid of 15;    c) cooling the fluid filaments in a water quench bath to form solvent-containing gel filaments;    d) removing the solvent from the filaments by extraction with trichlorotrifluoroethane; and    e) drawing the filaments in five steps before, during and after removing the solvent applying a draw ratio DRsolid of 36.5.
A high-performance polyethylene multifilament yarn is herein understood to mean a yarn containing at least 5 filaments made from ultra-high molar mass, or ultra-high molecular weight, polyethylene having an intrinsic viscosity (IV, as measured on solutions in decalin at 135° C.) of at least about 4 dl/g (UHPE), the yarn having a tensile strength of at least 3.0 GPa and a tensile modulus of at least 100 GPa (hereinafter also simply referred to as strength or modulus). Such HPPE yarns have a properties profile that make them an interesting material for use in various semi-finished and end-use products, like ropes and cords, mooring lines, fishing nets, sports equipment, medical applications, and ballistic-resistant composites.
Within the context of the present invention a yarn is understood to be an elongate body comprising multiple individual filaments having cross-sectional dimensions much smaller than their length. The filaments are understood to be continuous filaments; that is being of virtually indefinite length. The filaments may have cross-sections of various geometrical or irregular shapes. Filaments within a yarn may be parallel or entangled to one another; the yarn may be linear, twisted or otherwise departed from a linear configuration.
It is well known in the field of fibres and yarn technology that a multifilament yarn shows lower tenacity or tensile strength than the strength as measured on its constituent individual filaments. In general, the more filaments a yarn contains, the lower its tensile strength (breaking strength per unit of cross-sectional area, e.g. N/m2 or Pa).
FIG. 1 confirms the said decrease in tensile strength with increasing number of filaments in a yarn for some commercially available HPPE yarns; by showing tensile strength (TS) data for the indicated Spectra® and Dyneema® grades, as collected from brochures and web-sites of the respective producers and plotted versus the logarithm of the number of filaments (n) in the yarn. It is thus concluded that the strength of a multifilament yarn is always lower than that of its individual filaments.
It is furthermore well known, that spinning of high-strength multifilament yarn becomes increasingly difficult the higher the number of filaments in the yarn as spun, one of the likely reasons being differences in spinning and drawing conditions, and subsequently in properties, occurring between filaments. For a polyethylene multifilament yarn spinning process to be commercially viable on industrial scale, it is important that such process can be run continuously without interruptions and with high throughput rate, with a high number of filaments in the as-spun yarn.
In many of the above-mentioned applications, critical properties of HPPE yarn determining performance in use include tensile properties and creep behaviour. There is thus a constant need in industry for HPPE multifilament yarn showing improved performance, like improved tensile properties. Although various studies suggest the theoretical strength of a UHPE filament to be in the range 10-20 GPa, the strongest yarns available show much lower strength; for example a 780-filament Dyneema® SK75 yarn has a strength of about 3.5 GPa. More specifically therefore, there is a need for a process that enables production of such higher tensile strength yarn on industrial scale.
According to the present invention, this is provided by a process wherein in step b) each spinhole comprises a contraction zone with a gradual decrease in diameter from D0 to Dn with a cone angle in the range 8-75°, and wherein the spinhole comprises a zone downstream of the contraction zone of constant diameter Dn with a length/diameter ratio Ln/Dn of from 0 to at most 25, to result in a fluid draw ratio DRfluid=DRsp*DRag of at least 150, wherein DRsp is the draw ratio in the spinholes and DRag is the draw ratio in the air-gap, with DRsp being greater than 1 and DRag at least 1.
With the process according to the invention a HPPE multifilament yarn can be obtained that has higher tensile strength than any known HPPE yarn containing at least 5 filaments, especially an as-spun yarn; more specifically a HPPE multifilament yarn containing n filaments has and having a tensile strength TS obeying the formula TS≧f*(n−0.065) GPa, wherein factor f is at least 5.8 and n is at least 5.